This invention relates to glass-ceramics and, more particularly, to glass-ceramics for a substrate of an information storage medium. In particular, this invention provides glass-ceramics which have properties sufficient for use as a substrate of an information storage medium without using an arsenic component or an antimony component which adversely affects human beings and the environment.
This invention also offers a glass having properties sufficient for use as a substrate of an information storage medium without using an arsenic compound or an antimony component which adversely affects human beings and the environment.
In the specification of the present application, the term “information storage medium” means an information storage medium which can be used in stationary type hard disks, removable type hard disks and card type hard disks used as hard disks of a personal computer, hard disks for digital video cameras, digital cameras and audio devices, hard disks for car navigation systems, hard disks for mobile phones and hard disks for various electronic devices.
There has recently been an increased tendency to providing an information storage medium of a large capacity for coping with a multiple media tendency of a personal computer and for treating large data such as moving pictures animation and voice data in digital video cameras and digital cameras. As a result, bit density and track density of an information storage medium tend to increase and the size of bit cell tends to decrease for increasing the surface recording density of the medium. For this reason, the magnetic head is required to operate in a closer distance to the surface of a disk.
When the recording density exceeds 100 Gb/in2, thermal instability occurs in such a small magnetic domain and, therefore, the surface recording system can no longer cope with requirement for a high recording density exceeding 100 Gb/in2.
For coping with this problem, a perpendicular magnetic recording system has been adopted and for a large scale production of devices utilizing this system. In the perpendicular magnetic recording system, since the easy axis of magnetization runs in perpendicular direction, the bit size can be significantly reduced. Further, by having a desired film thickness of the medium (five to ten-fold of the film thickness in the surface recording system), reduction of demagnetizing field and an effect by shape magnetic anisotropy can be expected. For these reasons, the problems of decrease in the recording energy and occurrence of thermal instability in the conventional recording in the surface direction can be eliminated whereby a significantly higher recording density than in the surface recording system can be realized. Accordingly, in the perpendicular magnetic recording system, recording density of 100 Gb/in2 or over on a practicable level has already been achieved on a large scale production level and studies are being made about a recording density exceeding 300 Gb/in2.
This perpendicular magnetic recording system in which magnetization is performed in perpendicular direction relative to the surface of a medium is different form the conventional medium which has the easy axis of magnetization in the surface direction in that a medium having a magnetization easy axis running in perpendicular direction is used. As a recording layer of the perpendicular magnetic recording system, various alloy films including a barium-ferrite film, a Co-γFe2O3 alloy film, Co alloy films, Fe alloy films such as FePt film and Ni alloy films have been studied and put into practice.
It is however necessary in such magnetic recording medium to form the recording film at a high temperature for making crystal grains of the magnetic substance fine and causing the magnetic substance to grow in perpendicular direction. Besides, according to recent studies, it needs to be subjected to annealing at a high temperature in the order of 500° C. to 900° C. for improving its magnetic properties.
In order to cope with these requirements, there is a need for a substrate which is not likely to change its material configuration after annealing and which has sufficient heat resisting property. There is also a need for a substrate having very smooth surface flatness for enabling a low flying height of the magnetic head required by the tendency toward high recording density.
While there are requirements for a substrate having such properties and glass-ceramics which can realize such substrate, there is a need for a product which is free of an arsenic component which has been used as a refining agent of glass is likely to have an adverse effect to human beings and the environment and, for satisfying such need, an antimony component has been used as a refining agent.
It has recently been required, however, to reduce a harmful material further and there is a tendency to reducing or prohibiting use of a harmful material about which an adverse effect to human beings and the environment is considered to be relatively small. As a result, such tendency is applied also to the antimony compound which has been generally used as a material which is less harmful to human beings and the environment than the arsenic compound.
Japanese Patent Application Laid-open Publication No. 2001-76336 discloses a glass-ceramic substrate of an information storage medium comprising lithium disilicate as a predominant crystal phase and being free of arsenic and antimony compounds. Example Nos. 22 and 23 of this literature, however, have Young's modulus of 78 GPsa and 81 GPa and are insufficient in their mechanical strength which is currently sought for an information storage medium It is not known to the skilled artisan, therefore, whether or not there exist, in the glass-ceramics compositions of this literature, glass-ceramics which have properties including a sufficient refining effect and mechanical strength which are currently sought for an information storage medium. Such glass-ceramics have not actually been realized to date. Moreover, in the prior art glass-ceramics, amounts of SnO2 and CeO2 used as refining agents in the glass-ceramics are respectively 1 mol which exceeds 2.5% respectively when the amounts are converted to mass %. Although addition of such large amounts of SnO2 and CeO2 to the glass produces the refining effect in the molten glass, the specific gravity of the resulting glass-ceramics becomes large and, as a result, it becomes difficult to simultaneously satisfy both the high Young's modulus property and the low specific gravity property which are sought for an information storage medium.
Further, addition of a large amount of SnO2 or CeO2 gives rise to possibility of growth of a crystal phase which might hamper the properties sought for an information storage medium.
It is, therefore, an object of the present invention to provide glass-ceramics having properties suitable for use as a substrate of an information storage medium of next generation such as one for the perpendicular magnetic recording system without employing arsenic and antimony components which adversely affect human beings and the environment. Particularly it is an object of the invention to provide glass-ceramics for a substrate of an information storage medium having a coefficient of thermal expansion and chemical durability which match those of drive members, a low melting temperature and suitability for press forming which will enable high productivity.